The present invention relates to the electroless metallization of polyimide surfaces, more particularly to a method for improving electroless metal adhesion to a polyimide surface, and still more particularly to the electroless copper metallization of through-hole surfaces in multilayer printed circuit boards composed of polyimide insulating substrate material.
In the fabrication of multilayer printed circuits, i.e., printed circuits composed of a sandwich-like arrangement of alternating layers of circuitry and insulating substrate material, metallized through holes are provided in order to conductively interconnect the innerlayer circuitries as well as circuitry present on the outer-facing surfaces of the multilayer arrangement.
Ideally, the metal coating applied to the through hole surfaces should be complete in its coverage of the surfaces and should remain firmly adherent thereto throughout later fabrication steps, and in use of the printed circuit, so as to provide the desired electrical integrity in the completed printed circuit. To this end, and particularly for multilayer printed circuits in which the insulating material is epoxy-based (e.g., glass-reinforced epoxy resin), electroless metallization (e.g., electroless copper) is used to metallize the through holes, and steps are taken to prepare the through holes for complete and adherent receipt of the electroless metal deposit and electrical integrity among circuitry layers.
One such through hole preparation step is a so-called desmearing step, designed to remove from the exposed edges of the innerlayer circuitry epoxy resin smeared thereon as a consequence of the high-speed hole drilling operation and which otherwise would interpose a dielectric layer between those edges and the electroless metal deposit. Also known are so-called etch back procedures designed to maximize exposure of the innerlayer circuitry edges at the hole surfaces, as well as processes designed to improve the topography of the epoxy resin at the through hole surfaces so as to improve the ability of those surfaces to adherently receive the electroless metal deposit.
Reference may be had to U.S. Pat. Nos. 4,597,988 and 4,756,930 of Kukanskis, et al. in this regard, as well as Kukanskis, "Improved Smear Removal", CIRCUITS MANUFACTURING, March, 1983.
In recent years, polyimide and/or polyimide-containing materials have found increasing use as the insulating substrate material in printed circuits, offering excellent properties in terms of strength, thermal resistance, dimensional stability, and the like. At the same time, however, it has been recognized that these polyimide-based substrates are not particularly conducive to metallization, and in particular are difficult substrates on which to adhere an electroless metal deposit. Indeed, in recognition of this problem, IPC specifications for plated through holes in multilayer printed circuits employing polyimide substrates are quite relaxed, allowing up to 40% of the hole surface to exhibit hole wall pullaway (i.e., pulling away of the electroless metal deposit from the plated hole surface) so long as electrical integrity remains intact. Notwithstanding this foregiving specification, its attainment is often difficult to achieve and, more importantly, there is significant advantage to the fabricator and end user in achieving improved adhesion beyond that minimally specified.
Reference can be had to U.S. Pat. Nos. 4,842,946 and 4,873,136 of Foust, et al. for attempts to increase the adhesion of conductive metal to polyimide substrates, as well as to U.S. Pat. Nos. 4,725,504 and 4,832,799 to Knudsen, et al.